No volcanic ash has been detected, however, and the Civil Protection Department said all Icelandic airports remained open.

The country's meteorological agency said scientists were monitoring the eruption in the Holuhraun lava field, about three miles north of the Dyngjujoekull glacier.

"Visual observation confirms it is calm, but continuous," the weather agency said on its website.

This morning's eruption at about 0600 BST followed a smaller one in the same site on Friday that also prompted authorities to briefly raise the aviation warning code to restrict flights in the area. Thousands of small earthquakes have rocked the region in recent days, leading to concerns that the main volcano may erupt.

The red warning code - the highest in the country's alert system - meant that no flights are allowed in an airspace area of about 40 square nautical miles north of the fissure eruption area, up to 6,000 feet (1.1 miles) from the ground. Aviation officials said the restrictions do not affect commercial flights, which fly much higher than that.

Authorities said lava fountains of about 165ft (50m) high erupted from the fissure, estimated to be almost a mile long.

The fissure eruption appeared about 28 miles from the main Bardarbunga volcano, which lies under the vast Vatnajokull glacier that dominates the eastern corner of Iceland.

Though remote and sparsely populated, the area is popular with hikers in the summer. Officials earlier evacuated all tourists in the region after intense seismic activity there.

Although today's fissure eruption was more powerful than the one on Friday, experts say the situation is contained and is unlikely to result in the same level of aviation chaos as 2010.